But in order to the better and more precisely fixing upon what may be there seen, as the distinct species, of which all the shades of the retiring kind in nature partake, in some degree or other, the following scheme is offered, and intended as an additional means of making such simple impressions in the mind, as may be thought adequate to the four species of lines described in Chapter VII. Wherein we are to suppose imperceptible degrees of shade gradating from one figure to another. The first species to be represented by, 1, 2, 3, 4, 5. the second by, 5, 4, 3, 2, 1, 2, 3, 4, 5. and the third by, 5, 4, 3, 2, 1, 2, 3, 4, 5, 4, 3, 2, 1, 2, 3, 4, 5. gradating from the dots underneath, repeated either way. As the first species varies or gradates but one way, it is therefore least ornamental, and equal only to straight lines. The second gradating contrary ways, doubling the others variety, is consequently twice as pleasing, and thereby equal to curved lines. The third species gradating doubly contrary ways, is thereby still more pleasing in proportion to that quadruple variety which makes it become capable of conveying to the mind an equivalent in shade, which expresses the beauty of the waving line, when it cannot be seen as a line. The retiring shade, adequate to the serpentine line, now should follow; but as the line itself could not be expressed on paper, without the figure of a cone [Fig. 26 p. I], so neither can this shade be described without the assistance of a proper form, and therefore must be deferred a little longer. [Illustration: Fig. 26] When only the ornamental quality of shades is spoken of, for the sake of distinguishing them from retiring shades, let them be considered as pencilings only; whence another advantage will arise, which is, that then all the intervening mixtures, with their degrees of beauty between each species, may be as easily conceived, as those have been between each class of lines. And now let us have recourse to the experiments in life, for such examples as may explain the retiring power of each species; since, as has been before observed, they must be considered together with their proper forms, or else their properties cannot be well distinguished. All the degrees of obliquity that planes, or flat surfaces are capable of moving into, have their appearances of recession perfected by the first species of retiring shades, which may evidently be seen by setting opposite a door, as it is opening outwards from the eye, and fronting one light.